The present invention relates to a fixing device and to an image forming apparatus having therein the fixing device, and in particular to a fixing device of a belt type.
There has been known a fixing device of a belt type having a fixing roller, a belt that presses a recording material against the fixing roller and a pressing section that presses the belt against the fixing roller.
For avoiding deterioration of image quality, it is generally known about a fixing device that a nip pressure between a fixing roller and a belt needs to be increased in accordance with information relating to a recording material to be fixed (for example, a recording material thickness) or with fixing quality information (for example, the state of a gloss finished).
In the foregoing document, it is known that there is a possibility that, when pressurization by a pressing section is increased for the purpose of increasing a nip pressure between a fixing roller and a belt, a nip pressure on a central part is reduced compared with edge portions of the fixing roller in the longitudinal direction, and targeted image quality cannot be obtained.
For the aforesaid problem, there is known the first fixing device (for example, see Unexamined Japanese Patent Application Publication No. 2005-189746) wherein there is provided a cam (a pressure applying section) in which a prescribed circumferential area is made to be a standard surface which is away from the center axis at a constant distance in the longitudinal direction of a pressing section, and a specific circumferential area is made to be a non-standard surface whose central portion is further away than the standard surface in the longitudinal direction of a pressing section. When applying ordinary nip pressure, namely, pressing pressure, the standard surface side of the cam (pressure applying section) is pressed against a heat roller, while when applying a nip pressure higher than the ordinary pressure, the non-standard surface side is pressed while facing the heat roller. Owing to this, a nip pressure decline on the aforesaid central part which is generated when a pressure higher than an ordinary pressure is applied is tried to be reduced.
Further, there is known the second fixing device (for example, see Unexamined Japanese Patent Application Publication No. 2005-189746) wherein a plurality of springs movable in the direction perpendicular to the longitudinal direction are arranged on a pressing section in its longitudinal direction. when applying ordinary nip pressure, namely, the pressing pressure, the pressing section is pressed against a heat roller, by positioning plural springs at the same location in the direction perpendicular to the longitudinal direction of the pressing section, while when applying nip pressure that is higher than an ordinary pressure, the pressing section is pressed against a heat roller under the condition where springs positioned at a central part among the plural springs are moved toward the heat roller side, and thereby, a decline of nip pressure on the central part which is caused when a nip pressure that is higher than an ordinary pressure is applied, is tried to be reduced.
However, in the first fixing device described in Unexamined Japanese Patent Application Publication No. 2005-189746), a pressurizing member that is made of rubber whose thickness is about 1-3 mm, for example, is pressed by a pressure applying section (cam) directly, thus, there has been a possibility that pressing force of the pressurizing member may be changed greatly by an extremely small difference in a cam shape (for example, approximately 0.1 mm), and image defects caused by fixing may be generated.
There has further been a problem that design and production of a cam are extremely delicate, and a cam needs to be designed and produced again when a necessary nip pressure is not obtained.
In the second fixing device described in Unexamined Japanese Patent Application Publication No. 2005-189746, the aforesaid problems can be solved because the springs are used for pressurization. However, there has been a problem that there is a possibility that plural devices which move plural springs respectively in the direction perpendicular to the longitudinal direction of pressing section are needed for changing pressure distribution in the longitudinal direction of the pressing section, whereby an apparatus becomes complicated in structure and large in size and frequency of failures is increased.